Thread: date_trunc should be called date_round?

date_trunc should be called date_round?

From

Mario Splivalo

Date:

29 June 2009, 08:53:21

It's stated in the docs that date_trunc is "conceptually similar to the
trunc function for numbers.".

So, trunc(1.1) = 1, trunc(1.9) = 1, and so on.

But, date_trunc behaves like round function: round(1.9) = 2.

Example:

idel=# select date_trunc('milliseconds', '2009-01-01
12:15:00.000999+02'::timestamp with time zone);        date_trunc
----------------------------2009-01-01 11:15:00.001+01
(1 row)

fidel=# select version();                                                version

----------------------------------------------------------------------------------------------------------PostgreSQL
8.3.7on x86_64-unknown-linux-gnu, compiled by GCC
 
gcc-4.3.real (Ubuntu 4.3.3-5ubuntu4) 4.3.3
(1 row)


Or am I again completely misreading something?
Mike

Re: date_trunc should be called date_round?

From

Tom Lane

Date:

29 June 2009, 14:25:36

Mario Splivalo <mario.splivalo@megafon.hr> writes:
> But, date_trunc behaves like round function: round(1.9) = 2.

Hmm ... only for float timestamps, and only for the millisec/microsec
cases.
           case DTK_MILLISEC:
#ifdef HAVE_INT64_TIMESTAMP               fsec = (fsec / 1000) * 1000;
#else               fsec = rint(fsec * 1000) / 1000;
#endif               break;           case DTK_MICROSEC:
#ifndef HAVE_INT64_TIMESTAMP               fsec = rint(fsec * 1000000) / 1000000;
#endif               break;

I wonder if we should change this to use floor() instead.
        regards, tom lane

Re: date_trunc should be called date_round?

From

Mario Splivalo

Date:

30 June 2009, 06:52:03

Tom Lane wrote:
> Mario Splivalo <mario.splivalo@megafon.hr> writes:
>> But, date_trunc behaves like round function: round(1.9) = 2.
> 
> Hmm ... only for float timestamps, and only for the millisec/microsec
> cases.
> 
>             case DTK_MILLISEC:
> #ifdef HAVE_INT64_TIMESTAMP
>                 fsec = (fsec / 1000) * 1000;
> #else
>                 fsec = rint(fsec * 1000) / 1000;
> #endif
>                 break;
>             case DTK_MICROSEC:
> #ifndef HAVE_INT64_TIMESTAMP
>                 fsec = rint(fsec * 1000000) / 1000000;
> #endif
>                 break;
> 
> I wonder if we should change this to use floor() instead.
> 

I guess it's safe, since you don't have negative timestamps (right?) or
parts of timestamps (millisecs/microsecs), so floor() would act as trunc.

Esp. if for the other parts of timestamp (days, hours, ...) it's actualy
truncating, not rounding, i.e.:

date_trunc('minute', '2009-01-01 12:13:50'::timestamp)

would return '2009-01-01 13:13:00', not '2009-01-01 13:14:00'.

One would expect similar behavior for the milli/microsec part.

Now it's truncating, unless dealing with milli/microseconds, where it's
rounding.
Mike